What is the meaning of Ullage?

In a wine bottle, the empty space between the cork and the top of the wine.

In a cask or barrel, the empty space, occupied by air, that is created by not completely filling the cask or barrel, or through spillage.

The topping-up of such a barrel with fresh wine.

In an industrial setting, the empty space in a tank, especially as for fuel.

Additional cargo of little or no value taken on to prevent movement of shifting of the purposive cargo.

Left-over wine surreptitiously drunk by waiters as they clear away the glasses.

To gauge the amount of empty space between the top of a cask and the level of liquid inside it.

Source: wiktionary.org